PURPOSE: Diagnosis of pneumothorax relies on clinical suspicion and chest X-ray, and is often delayed. We aimed to determine whether electrical impedance tomography (EIT) can accurately identify the presence of surgically created pneumothoraces before significant changes in clinical parameters. METHODS: Six anesthetized and muscle-relaxed piglets with surfactant-depleted lungs were studied. Following chest drain insertion into the right ventral chest, 10-20 ml aliquots of air were instilled into the pleural space to a maximum volume of 200 ml. The pneumothorax was drained by attaching a Heimlich valve to the chest drain. At each instillation and after draining the pneumothorax, global and regional end-expiratory intra-thoracic volumes (EEV) were measured using respiratory inductive plethysmography (RIP) and EIT concurrently with [Formula: see text], heart rate and blood pressure. RESULTS: A significantly greater change in both global EEV(RIP) and EEV within the right ventral quadrant was seen at all volume instillations, from as little as 10 ml, compared with all other quadrants. There was no difference in EEV within the left ventral and both dorsal quadrants. [Formula: see text] fell below 90% at 100 ml instillation. Tachycardia occurred at 140 ml instillation. EIT identified a 60% resolution of pneumothoraces within 60 s of attachment of the Heimlich valve. CONCLUSIONS: EIT accurately detects very small pneumothoraces before physiological parameters change.